Characteristics of the global thermal tropopause derived from multiple radio occultation measurements

Thermal tropopause represents the region of the atmosphere where the environmental lapse rate changes from tropospheric positive to stratospheric negative. It also defines the altitude of the atmosphere beneath which significant weather perturbations occur except occasional overshooting thunderstorms in the tropical regions. Accordingly, how the temporal and spatial variability of thermal tropopause behaves is of great concern in atmospheric research and, hence, investigated in this study by using radio occultation (RO) observations obtained from seven space missions during the period from May 2001 to April 2013 (with a total of 6,075,359 occultations). While RO observations have been demonstrated to provide precise measurements of temperature profiles of the atmosphere, their results are inter-compared before further use in our analysis, showing expected high-precision observations with mean differences < 0.06 K and standard deviations < 1.6 K in the upper troposphere and lower stratosphere. Given a rather large data set of multi- space-mission RO measurements taken globally, a very detailed description of spatial structure and variability of the tropopause is revealed, and monthly mean zonal mean tropopause parameters in each 2° latitude band from 90°S to 90°N can be obtained. Many interesting features of seasonal cycle, spatial distribution, interannual variation, and diurnal variation of the thermal tropopause are observed. For examples, except for the primary minimum in January, the equatorial tropopause temperature exhibits a secondary minimum in April, possibly caused by the strongly combined wave forcing from two hemispheres; During the boreal winter over the tropics, the distribution of tropopause temperature extrema do not totally coincide with the altitude extrema spatially, and the former has a better agreement with the locations of strong tropical convection systems; Notable zonal asymmetries in interannual characteristics are observed in both tropical and extratropical regions. In both the tropics and Arctic, close correlation of the interannual variations is revealed between tropopause parameters and stratospheric temperatures in localized regions as well as zonal mean results while no such relationship is observed in the middle latitudes; and Diurnal variation of the equatorial tropopause shows warmer temperature in the morning and cooler value at midnight.